Please pay attention that the urbackup database is located under /var/urbackup and also writes much onto your USB stick and can fill up your file system.
That is the urbackup user's home directory. I have a feeling that if you stopped the service, usermod -m -d /path/to/new/homedir urbackup, and restarted the service, it would still work. That directory would need executable permissions though. So, a normal OMV created data drive would not work.